1. Field of the Invention
The present invention relates to vehicle detectors using loop sensors, and in particular relates to a vehicle detector using a loop sensor which includes a frequency change detector for detecting vehicles at low speeds.
2. Description of Related Art
Below described first are vehicle detectors using loop sensors of prior art.
FIG. 1 is a diagram of a vehicle detector using a loop sensor of prior art.
A vehicle detector using a loop sensor illustrated in FIG. 1 detects a vehicle""s passing, from changes in the resonant frequency caused by changes in the loop inductance which occur when a conductor passes over the loop (11). In order to detect changes in the resonant frequency and to output vehicle detection signals, such vehicle detector uses a PLL (12) and a detection circuit (13). PLL""s operation is explained below.
FIG. 2 is a block diagram of a PLL. PLL (Phase-Locked Loop) is an oscillator which can trace input signals through a closed loop control, in order to prevent phase differences between outputs from an oscillator and input signals. Outputs of the oscillator thus are synchronized to input signals. The PLL consists of a phase detector (21), a low-frequency pass filter (22), and a voltage control oscillator (24). First, the phase detector (21) compares phases of two input signals and generates a voltage proportional to the phase difference between the two signals, which is outputted as a voltage representing the difference between two frequencies after going through the low frequency pass filter (22) and the amplifier (23).
The voltage control oscillator (24) generates output frequencies based on the voltage input generated above, and such operations are repeated until there is no frequency difference between the output frequencies and the input signals.
When a loop sensor is connected to the input and output ports of a PLL and the normal state of no vehicle passage is maintained, input and output frequencies are synchronized to xcfx89 FR, and V1, V2, and V3 all have value xe2x80x980.xe2x80x99 At this state, if a vehicle passes through the sensor and thus there is a sudden increase in the input frequency xcfx89 i, an output voltage V3 is generated, and xcfx89 0 increases according to the characteristic of the voltage control oscillator (24). The loop goes into an equilibrium state with such increased frequency. On the other hand, if the input frequency decreases, V3 is changed to be a negative value and xcfx89 0 decreases according to the characteristic of the voltage control oscillator (24). The PLL again goes into an equilibrium state with such decreased frequency, and thus goes into a phase-locked state.
Vehicle detectors using such loop sensors of prior art may recognize a vehicle of a low speed to be a number of different vehicles. A vehicle at a low speed may change its speed while it is within the range of a loop sensor, by repeating acceleration and deceleration after its entry into and before exit from the loop sensor. Such a vehicle passing over a loop sensor at a low speed may cause the loop sensor to output signals which look same to signals generated by a number of vehicles passing at high speeds, and thus may impair accurate detection. FIG. 3 illustrates an example of such an error of recognizing a vehicle of a low speed which repeats accelerating and deceleration while on the loop sensor, to be a number of different vehicles.
If a vehicle enters a loop sensor, it is detected through the change in frequencies caused by such an entry of a vehicle. For a vehicle of a low speed, if the vehicle decelerates its speed even a little bit before exiting the loop, the operation of the re-equilibrated loop is stopped and one vehicle passage is detected. After this, if the vehicle accelerates again, another detection is made for such change. Detection for such changes in speed can be made until the vehicle makes complete exit from the loop. In the example of FIG. 3, a low-speed vehicle was detected to be three vehicles.
Because vehicle detectors can be useful especially in adverse traffic conditions, such as a bumper-to-bumper condition, it is essential to solve the above problems which can arise when a vehicle of a low speed passes over a it loop sensor.
It is an object of the present invention to provide vehicle detectors using loop sensors which can accurately detect vehicles passing over loop sensors at low speeds, in order to solve the above problems presented by prior art.
In accord with the above object, a vehicle detector using a loop sensor by the present invention comprises a loop sensor having different resonant frequencies according to changes in the loop""s inductance caused by passing vehicles, a PPL for outputting vehicle detection signals upon detecting changes in resonant frequency of the said loop sensor, a frequency change detector connected parallel to the said PLL, a logic circuit whose outputs are generated using the signals from the said PLL and the said frequency change detector, and a micro-processor which determines the vehicle detection based on outputs from the said PLL and the said frequency change detector.
In a vehicle detector using a loop sensor of the present invention, the said frequency change detector can be implemented with a frequency/voltage converter.
If a frequency change detector of a vehicle detector using a loop sensor by the present invention is implemented with a frequency/voltage converter, vehicle detection signals to be used are outputs from logical OR operations or other logical ones of the said PLL""s outputs and frequency/voltage converter""s outputs.
In a vehicle detector using a loop sensor of the present invention, the said frequency change detector can be implemented with a frequency counter.
If a frequency change detector in a vehicle detector using a loop sensor of the present invention is implemented with a frequency counter, PLL""s signals are made to be external interrupt signals to the micro-processor, in order to enable the micro-processor to examine outputs from the frequency counter only when there is a response from the PLL.